As a packaging method of an electronic device such as an acoustic wave device, there has been known a method that face-down mounts a chip on a circuit board and covers the periphery of the chip with a sealing member. The integration and downsizing of the acoustic wave device have been desired. Japanese Patent Application Publication No. 2008-546207 describes that two substrates each including an acoustic wave element formed on the surface thereof are bonded together through an interlayer so that the acoustic wave elements face each other across an air gap.
In the method that stacks substrates each including a functional element such as an acoustic wave element formed thereon as disclosed in Japanese Patent Application Publication No. 2008-546207, the stress between the substrates becomes a problem. For example, the thermal stress between the substrates strains the substrates, deteriorating the characteristics of the functional elements.